WINTER 16 EXAMINATION Model Answer Subject Code: 17657 Important Instructions to examiners: 1) The answers should be examined by key words and not as word-to-word as given in the model answer scheme. 2) The model answer and the answer written by candidate may vary but the examiner may try to assess the understanding level of the candidate. 3) The language errors such as grammatical, spelling errors should not be given more Importance (Not applicable for subject English and Communication Skills. 4) While assessing figures, examiner may give credit for principal components indicated in the figure. The figures drawn by candidate and model answer may vary. The examiner may give credit for any equivalent figure drawn. 5) Credits may be given step wise for numerical problems. In some cases, the assumed constant values may vary and there may be some difference in the candidate s answers and model answer. 6) In case of some questions credit may be given by judgement on part of examiner of relevant answer based on candidate s understanding. 7) For programming language papers, credit may be given to any other program based on equivalent concept. 1. a) Attempt any THREE of the following: 12 i) Draw block diagram of basic cellular system. State advantages of cellular system. Ans:- ( Diagram- 2 mks, any four advantages- 2 mks) Fig: Block diagram of basic cellular system Advantages of cellular system: 1. Higher capacity-smaller the size of the cell more the number of concurrent users i.e. huge cells do not allow for more concurrent users. 2. Less transmission power-huge cells require a greater transmission power than small cells. Page 1 of 30
3. Local interference only-for huge cells there are a number of interfering signals, while for small cells there is limited interference only. 4. Robustness-As cellular systems are decentralized, they are more robust against the failure of single components. ii) List the following specifications of EDGE 2.5 G standard. (1) Backward compatibility (2) Channel bandwidth (3) Data rate (4) Duplexing method Ans: ( Relevant answer 4 mks) Specifications of EDGE 2.5 G standard 1) Backward compatibility GSM 2) Channel bandwidth 200 KHz 3) Data rate 547.2 kbps 4) Duplexing method - FDD iii) Compare IS-95B standard with GPRS with respect to following points. (1) Backward compatibility (2) Channel bandwidth (3) Duplexing Tech (4) No. of voice channels Ans:- ( Relevant comparison points- 4 mks) IS-95B GPRS Backward compatibility IS.95 GSM Channel bandwidth 1.25 MHz 200 KHz No of voice channels 64 8 per carrier Duplexing method FDD TDD iv) Illustrate operation of cordless telephone system with the help of neat sketch. Ans: ( Block diagram- 2 mks, operation- 2 mks) Page 2 of 30
OR OR Cordless Telephone Systems are full duplex communication systems that use radio to connect a portable handset to a dedicated base station, which is also connected to a dedicated telephone line with a specific telephone number on PSTN. Page 3 of 30
The fixed port of a cordless telephone is nothing but the base unit on which cordless handset is placed, is connected to a telephone line and an adapter to produce a dc supply for various electronic circuits inside the base unit. The communication between the base unit and the handset is wireless and the range is limited to 50 meters. In the base unit all call processing circuits like amplifiers and also ring circuit is present. In addition a transceiver is also present which is used for communication with the handset. In handset also the transceiver along with an antenna, amplifier, microphone and loud speaker are present. 1 st generation cordless telephone systems could cover only distance of a few ten meters(approximately 50m)and can be operated solely as extension telephones to a transceiver connected to a subscriber line on the PSTN and are primarily for in-home use. 2 nd generation cordless telephone systems could cover distance of a few hundred meters which allows subscribers to use their handsets at many outdoor locations within urban centers. Cordless telephone systems provide the user with limited range and mobility, as it is not possible to maintain a call if the user travels outside the range of the base station. b) Attempt any ONE of the following: 06 i) Describe various stages of call processing in GSM system with the help of neat diagram. Ans: ( Block diagram- 3 mks, operation-3 mks) Page 4 of 30
(OR) Page 5 of 30
Fig: GSM Operations from speech input to output Source Coding Source Encoding is a technique in which the encoder converts the digital input signal into another digital signal. It is used to reduce or eliminate redundancy for ensuring an efficient representation of source output. Various source coding techniques are: ASCII(American Standard Code for Information Interchange) EBCDIC(Extended Binary Coded Decimal Interchange Code) Baudot Code Channel Coding Channel coding is performed at the transmitter in order to minimize the effect of channel noise. It also converts the input signal in such a form which is compatible with the channel used for communication. Channel coding techniques add more bits into the message bits. These additional bits serve the purpose of error detection and correction. But these bits do not contain any information and therefore these bits are called as redundant bits. Interleaving Total of 456 data bits can be transmitted within each 20mS speech frame or control message frame. This 456 data bits are broken into eight 57 bit sub-blocks. These eight sub-blocks which make up a single frame are spread over a 8 consecutive Traffic Channel(TCH) time slots. Ciphering It modifies the contents of the eight interleaved blocks through the use of encryption techniques known only to the particular mobile station and base transceiver station. Two ciphering algorithms A3 and A5 are used in GSM to prevent unauthorized network access and privacy for the radio transmission respectively. Burst Formatting It adds binary data to the ciphered blocks, in order to help synchronization and equalization of the received signal. Page 6 of 30
ii) Illustrate the process of call initiation from landline telephone to cellular phone with neat timing diagram. Ans: When a cellular phone is turned ON, but not yet engaged in a call, it first scans the group of forward control channels to determine the one with the strongest signal, and then monitors that control channel until the signal level drops below a usable level. Call initiation by a landline (PSTN) subscriber to mobile user: 2 marks The mobile switching centre (MSC) dispatches the request to all base station in a cellular system. The Mobile Identification Number (MIN) which is subscriber telephone number is then broadcast as a paging message over all of the forward control channels throughout the cellular system. The mobile receives the paging message sent by BS which s monitors, and responds by identifying itself over the RCC. The BS relays the acknowledgement sent by the mobile and informs the MSC of handshake. The MSC instructs the BS to move the call to an unused voice channel pair within the cell. The BS signals the mobile to change frequencies to an unused forward and reverse voice channel pair. Another data message is transmitted on forward channel to instruct the mobile telephone to ring and mobile user to answer the phone. Figure below shows sequence of events involved in call connection. Page 7 of 30
2. Attempt any FOUR of the following: 16 a) Describe the effect of co-channel interference in cellular systems. How it affects system capacity? Ans: ( effect- 2 mks, effect on system capacity 2mks) Page 8 of 30
b) State any four specifications of UMTS. Ans:(any four- 4 mks) Specifications of UMTS: c) Name the systems A and B which supports following features. Parameter A Parameter B 1) Frequency band 2400-2483.5 MHz 1) Frequency band 384 kbps 2) Duplexing method with frequency hopping TDD 2) Duplexing method TDD 3) Channel BW -1 MHz 3) Channel BW 1.6 MHz 4) Modulation tech GFSK 4) Modulation tech Smart antenna Ans: System A is Bluetooth. System B is 3G TD-SCDMA d) List four specifications of GPRS 2.5G GSM standard. Ans: Specifications of GPRS 2.5G GSM standard:- ( any four- 4 mks) Backward compatible GSM. Channel BW 200 KHz Data rate 171.2 kbps No. of voice channels 8 per carrier e) State four features of Bluetooth. Page 9 of 30
Ans: Features of Bluetooth: (any four -4 mks) f) Describe how umbrella cell approach is used in cellular system to reduce the number of hand-offs? Ans: ( diagram- 2 mks, explanation- 2 mks) Explanation-Capacity of cellular system can be increased using additional cell sites but it is practically difficult to get new physical site in urban areas. Thus instead of new cell sites additional channels and base stations can be installed to increase the capacity. Different antenna heights and different power levels can be used to provide small and large cells located at single site. This technique is called as umbrella cell approach. This approach is used to provide large area coverage to high speed users while small area coverage to low speed users. It also ensures additional microcell channels for pedestrian users. The speed of each user is estimated by base station or MSC by evaluating how rapidly the short term average signal strength on RVC changes over time or with sophisticated algorithms. If high speed user in the large umbrella cell is approaching the base station, and its velocity is rapidly decreasing, the base station may decide to hand the user into the co-located microcell without MSC permission. This approach is basically used to reduce number of hand off for high speed users. 3. Attempt any FOUR of the following: 16 a) Draw block diagram of frequency synthesizer unit of mobile handset and state it s function in cellular handset. Ans:- (block diagram- 2mks, function-2 mks) Page 10 of 30
Fig: block diagram of frequency synthesizer unit of mobile handset The synthesizer is used for developing all the signals used by the transmitter and receiver. It uses the PLL circuits and a mixer. The crystal oscillator provides a reference for the two PLLs. The output of VCO-2 is used as a local oscillator frequency for the first mixer in the receiver. The outputs of the two VCOs are mixed together to produce the transmitter output frequency. The frequency divider block receives the divide by numbers from the logic section. These numbers are given by the MTSO computer. The divide by numbers will set the transmitting and receiving channel frequencies. The two outputs produced by the frequency synthesizer are applied to the modulator box in the transmitter and the first mixer in receiver respectively. Thus the frequency synthesizer acts a local oscillator which can produce a wide range of frequencies with high stability. b) Draw GSM architecture and explain function of HLR and VLR. Ans( diagram- 2 mks, each function- 1 mks) Page 11 of 30
c) State the different techniques used to improve capacity and coverage in cellular system. Describe cell sectoring technique in detail. Ans:( 4 methods- 2 mks, Sectoring explanation 2 mks) Different techniques used to improve capacity and coverage in cellular system are: 1) Cell sectoring 2) Cell splitting 3) Microzone cell concept 4) Repeaters Cell Sectoring Page 12 of 30
Decrease the co-channel interference and keep the cell radius R unchanged Replacing single Omni-directional antenna by several directional antennas Radiating within a specified sector The process of reducing the co-channel interference and thus increasing the capacity of the system by using directional antennas is known as Sectoring Cell increases the number of channels per unit area By decreasing the cell radius R and keeping the co-channel reuse ratio D/R unchanged Another way to increase capacity is to keep the cell radius unchanged and decrease D/R ratio. Sectoring increases SIR so that the cluster size may be reduced. First the SIR is improved using directional antennas, and then capacity improvement is achieved by reducing the number of cells in a cluster. So, increasing the frequency reuse it is necessary to reduce the relative interference without decreasing the transmit power The co-channel interference in a cellular system may be decreased by replacing a single omnidirectional antenna at the base station by several directional antennas, each radiating within a specified sector The factor by which the co-channel interference is reduced depends on the amount of sectoring used A cell is normally partitioned into three 120 sectors or six 60 sectors as shown in figure, In sectoring, the channels used in a particular cell are broken down into sectored groups and are used only within a particular sector. Assuming seven-cell reuse, for the case of 120 sectors, the number of interferers in the first tier is reduced from six to two. This is because only two of the six co-channel cells receive interference with a particular sectored channel group. Disadvantages: Page 13 of 30
Increased number of antennas at each base station As sectoring reduces the coverage area of a particular group of channels, the number of handoffs increases. d) List any four features of third generation (3G) cellular standard. state various 3G standards. (TDMA and CDMA based). Ans: ( four features- 2mks, four standards- 2 mks) 1. The main feature of 3G technology is that it supports greater voice and data capacity and high data transmission at low-cost. 3G mobiles can operate on 2G and 3G technologies. 2. The second major feature is the security: 3G offers greater security features than 2G like Network Access Security, Network Domain Security, User Domain Security, Application Security. 3. This technology provides localized services for accessing traffic and weather updates. Video calls and video conference is another major feature in 3G mobile technology. These features reduces the communication barriers between people, that were not sacked even with mobile phones. 4. Date transfer rates are high and can support even live TV channels over phone. 5. Online media is another exciting feature in 3G mobiles. 6. 3G mobiles highly attract the music lovers as they can listen to music and watch videos online and can download huge files with in less time. Standards- 1) CDMA 2000 2) WCDMA-UMTS 3) 3GTD-SCD 4) IMT2000 e) State and explain the various performance services offered by SS7 protocol. Ans: ( any four 4 mks) Page 14 of 30
4. a) Attempt any THREE of the following: 12 i) State the radio spectrum for IMT-2000. State vision of FMT-2000 (four points) Ans:( any four spectrum- 2 mks, any four vision- 2 mks) Spectrum- Page 15 of 30
Vision- OR ii) State the various services offered by GSM standard. Describe these services in detail. Ans: ( any four services- 2 mks, description 2 mks) Page 16 of 30
Telephone Services: Teleservices include Standard mobile telephone Mobile-originated Base-originated traffic. emergency calling Fax Videotext Tele text, SMS MMS. Supplementary ISDN services: This service are digital in nature and include Call diversion Caller line ID Closed user group Call barring Call waiting Call hold Connected line ID Multiparty (Teleconferencing) Call charge advice This service also include the Short Messaging Service (SMS) which allow SM subscriber and BS to transmit alphanumeric pages of limited length (160-7 ASCII characters) while simultaneously carrying normal voice traffic iii) Compare GPRS with IS-136. (Any four points) Ans: (any four- 4 mks) GPRS IS-136 Date rate 171.2 kbps Channel BW 200 KHz 30 KHz No. of voice channels 8 per carrier 3 Multiple access method TDMA TDMA Modulation technique GMSK and 8 PSK π/4 DQPSK iv) Define the term adjacent channel interference. State methods to reduce it? Ans:-Defination with diagram- 2 mks, methods (two )- 2 mks each) Page 17 of 30
b) Attempt any ONE of the following: 06 i) Describe the concept of frequency reuse used in cellular systems. Also calculate the capacity for cluster size of 7 in cellular system which has 504 radio channels available for handling traffic. Calculate number of channels per cell. If number of clusters available in cellular systems are 15, then calculate capacity of system. Ans: ( frequency reuse-3 mks, solution 3 mks) Page 18 of 30
Number of channel = 504 Cluster size = 7 Capacity = M x K x N No. of channels per cell = 504/7 For no. of clusters = 15 Capacity = 15 x 72 x 7 = 7560 = 72 ii) Draw SS7 protocol architecture and state the function of NSP of SS7. Ans: ( Diagram- 3 mks, Function- 3 mks) Page 19 of 30
Function of NSP:- Network Service Part of SS7: The NSP provides ISDN with a highly reliable and efficient means of exchanging traffic using connectionless services. The NSP consists of Message transfer Part(MTP), Signaling Connection Control Part(SCCP). The function of MTP is to ensure that signaling traffic can be transferred and delivered reliably between the end-users and the network. Signaling data link functions (MTP Level 1): This level provide an interface to the actual physical channel (copper wire, fiber, satellite link etc) over which communication takes place. Signaling link function (MTP Level 2): It provides a wide range of error detection and correction features. Signaling Network Function (MTP Level 3): Provides procedure that transfer message between signaling nodes. It has two functions namely, Signaling Message Handling(SMH) and Signaling Network Management(SNM). SMH is used to provide routing, distribution and traffic discrimination. Signaling Connection Control Part (SCCP): The SCCP provides enhancement to the addressing capabilities provided by the MTP. Page 20 of 30
5. Attempt any FOUR of the following: 16 a) Draw neat block diagram of transmitter unit of mobile handset. State function of APC loop and duplexer unit in unit. Ans: - ( block diagram-2 mks, function 1 mks each) OR APC- Page 21 of 30
Duplexer b) Compare GSM standard with N-Amps standard with respect to following point. i) Generation ii) Channel bandwidth iii) Whether analog or digital iv) Data rate v) Frequency band. (Any four points) Ans: ( any four- 4 mks) GSM N-Amps Generation 2 G Before 2 G Channel bandwidth 200 KHz 10 KHz Whether analog or digital Digital Analog Data rate 270.833 Kbps Frequency band 1.85 1.99 GHz 824-894 MHz Page 22 of 30
c) Draw the forward channel structure of IS-95. Write function of each channel in it. Ans:( structure- 2 mks, function- 2 mks ) Page 23 of 30
d) Draw neat block diagram of wireless local loop (WLL) network and state its importance. Ans: - (diagram- 2 mks, importance- 2 mks) Importance:- e) State importance of the following terms:- i) Blockage ii) Voice call blockage Page 24 of 30
iii) Voice quality iv) Word error rate Ans: ( each definition 1 mks) f) Define the following terms. i) Control channels ii) Mobile station iii) MSC iv) Cell splitting Ans: (each definition 1 mks) Page 25 of 30
6. Attempt any FOUR of the following: 16 a) Complete and identify the given block diagram and state function of block A and RSSI signal. Ans: Complete diagram- 1 mks, identification- 1 mks, function of block A and RSSI Signal 1 mks each) Complete diagram- Identification-The given circuit is cellular receiver Page 26 of 30
Function of- Block A- Duplexer-The transmitter output is fed to a duplexer or isolator which allows the transmitter and receiver and receiver frequencies to share the same antenna. RSSI Signal- b) Compare IS-95 standard with GSM system with respect to following points. i) Channel bandwidth ii) Type of modulation iii) SMS length iv) Number of voice channels. Ans: (Relevant comparison- 4 mks) Points GSM IS-95 Channel bandwidth 200 KHz 1.25 MHz Type of modulation GMSK QPSK /BPSK SMS length 160 120 Number of voice channels 8 per channel 64 per channel c) Draw 4G wireless architecture and state any four features of 4G standard. Ans: -( Architecture( any relevant diagram) - 2 mks, any 4 relevant features- 2 mks) Page 27 of 30
Features of 4G: Fig: 4G Wireless Architecture Page 28 of 30
d) Draw block diagram of mobile unit and state function of each block. Also state two features of mobile hand set. Ans:( Diagram- 1 mks, function of each block-2 mks, any two features-1 mks) Features- 1) Typical o/p power is 3 W if mobile unit is mounted on vehicle 2) o/p power is only 500Mw if it is a handheld unit 3) transmitter is a low power FM unit operating in frequency range of 825 to 845 Mhz 4) It has 666 transmit channels which are spaced 30 Khz apart Page 29 of 30
e) List out any four key features of IS-95 CDMA system. Ans: (Any Four relevant key features- 4 mks) Key features of IS-95 CDMA system: Page 30 of 30